Songbirds are one of the few animal models for human speech in which it is possible to experimentally investigate basic physiological and acoustic challenges associated with producing complex, learned vocalizations. Parallels between the production of speech and birdsong have recently been strengthened by the discovery in birds of new song-related motor programs that modulate the dimensions of the supra glottal vocal tract, causing it to act as a variable resonance filter that tracks the fundamental frequency of sound generated in the vocal organ. The following research focuses on sensorimotor control of the oropharyngeal-esophageal cavity (OEC), which dominates the vocal tract filter. A long-term goal is to increase our understanding of how the nervous system controls complex behaviors. The first specific aim is to perform a detailed functional morphological analysis of the songbird head and neck, using micro dissection, MRI and CT scan. The data obtained will be combined with that from cineradiography of vocal tract movements during song. The resulting kinematic model will be elaborated by the addition of in vivo and in vitro measures of biophysical, physiological and biochemical factors determining the contractile performance of the relevant vocal tract muscles. The inclusion of these data in the model will make it possible to convert the kinematic model into a dynamic 3D biomechanical model of song-related vocal tract movements. The second specific aim investigates the role of sensory feedback in the OEC's ability to keep its resonance frequency tuned to the song's fundamental frequency, which is controlled by the syrinx. Cineradiography will be combined with manipulation of sensory feedback by deafening, pitch shifting vocal output or altering the fundamental frequency to determine the role of sensory feedback in controlling vocal tract filters so their resonance matches the fundamental frequency. The possible role of somatosensory feedback in tuning the vocal tract filter will also be investigated. The third specific aim will use neuroanatomical and neurophysiological techniques to map the premotor pathways of the cranial nerves in order to identify the neural pathways by which auditory information accesses the motoneurons to muscles of the upper vocal tract and to look for possible neural connections to the brain's song control nuclei, which controls the vocal organ. Songbirds provide a valuable model system in which to investigate some of the control mechanisms relevant to various speech pathologies that are important to public health.